OFDM communication has been chosen for most of the modern high-data rate communication systems (Digital Audio Broadcast—DAB, Terrestrial Digital Video Broadcast—DVB-T, and Broadband Radio Access Networks—BRAN such as HIPERLAN/2, IEEE802.11a/g, IEEE802.15.3a, for example, and is considered for future wide-band telephony standards, referred to as “4G”). However, in most cases the receiver needs an accurate estimate of the channel impulse response. Also, in the context of wireless local area networks (‘WLANs’), current data rates (54 Mbps on top of the physical layer) are foreseen to be insufficient for very dense urban deployment, such as for hot spot coverage. This is the motivation for IEEE to propose and specify in the scope of the IEEE802.11n (the former High Throughput Study Group) solutions for very high data rate WLANs (targeting at least 100 Mbps on top of the medium access control (‘MAC’) layer) in the 5 GHz band. Another area of investigation is that of higher frequency bands where more spectrum is available such as the 60 GHz band.
One way of achieving higher data rates is to provide the system with multiple antennas both at the transmitter and at the receiver. By doing so, it is possible to increase the quality of the communication link by exploiting the spatial diversity dimension using for instance Space Time Block Codes (‘STBC’), or to increase the spectral efficiency of the system by transmitting simultaneously different streams using Spatial Division Multiplexing. Therefore, Multiple Transmit Multiple Receive (MTMR) antenna systems are strong candidates for next generation WLANs and certain other OFDM communication systems.
In the so-called Cyclic Prefix OFDM (CP-OFDM) modulation scheme, each OFDM symbol is preceded by a guard interval that is longer than the channel impulse response (CIR) and a cyclic prefix or postfix, hereinafter referred to collectively as a cyclic affix, is inserted at the transmitter in a guard interval between consecutive OFDM symbols, the cyclic affix consisting of samples circularly replicated from the useful OFDM symbol time domain samples. The cyclic affix enables very simple calculation for the equalisation at the receiver, where the cyclic affix is discarded and each truncated block is processed, for example using Fourier Transform (usually Fast Fourier Transform (FFT)), to convert the frequency-selective channel output into parallel flat-faded independent sub-channel outputs, each corresponding to a respective sub-carrier. For equalisation purposes, numerous strategies exist. Following the zero forcing approach, for example, each sub-channel output is, unless it is zero, divided by the estimated channel coefficient of the corresponding sub-carrier.
In the Zero Padded OFDM (ZP-OFDM) modulation scheme, as described for example in the article by B. Muquet, Z. Wang, G. B. Giannakis, M. de Courville, and P. Duhamel entitled “Cyclic Prefixing or Zero Padding for Wireless Multicarrier Transmissions” IEEE Trans. on Communications, 2002, the cyclic affix is replaced by null samples. This solution relying on a larger FFT demodulator, has the merit to guarantee symbol recovery irrespective of channel null locations when the channel is known.
However channel estimation and tracking remains an issue, especially in the presence of high mobility or high frequency and data rates. Like other digital communication systems, OFDM modulation encounters problems at high Doppler spreads, which occur notably when the user is moving fast, for example in a car, or even at pedestrian speeds when investigating the area of higher frequency bands where more spectrum is available such as the 60 GHz band. Accordingly, the channel impulse response needs to be constantly tracked and updated, especially in the presence of high Doppler spreads.
It would be desirable for the OFDM modulation system to keep all the advantages of classic OFDM and additionally allow very simple and (semi-)blind channel estimation at the receiver. Semi-blind channel estimation means that substantially no additional redundancy is added to the system with respect to classic CP-OFDM, and therefore no bandwidth for data transmission would be lost; however, semi-blind channel estimation can be realized thanks to deterministic sequences known at both the transmitter and the receiver sides, as long as there is no substantial bandwidth loss for data transmission. Such a system would be advantageous in low-mobility scenarios and would make OFDM systems applicable to high-mobility scenarios as well.
Our co-pending European Patent Application EP 02 292 730.5 describes a communication method in which the CP-OFDM time domain redundancy is replaced by a pseudo-randomly weighted deterministic sequence which leads to the so called Pseudo Random Postfix OFDM (PRP-OFDM). The advantages of being able to use ZP-OFDM are preserved and low complexity channel estimation at the receiver is made possible. Note that PRP-OFDM does not impact the achieved useful data rate and spectral efficiency compared to the classical CP-OFDM modulator, apart possibly from transmission of small amounts of data for the calculation of pseudo random parameters, since the only modification is the affix content, thus the low complexity channel estimation possible at the receiver side is also semi-blind.
Our co-pending European Patent Application describes the application of PRP-OFDM to single transmit antenna systems and it is desirable to apply comparable techniques to MTMR systems, capable of using more than one transmit and/or receive antenna.